Refactor coding system and mapping Downloader classes

[Jongmassey]

Following this comment on a PR review.

The bnf to dmd mapping downloader was based on the TrudDownloader class used by the dm+d and SNOMED CT coding systems for reasons of expediency and the ease of integrating it into the scheduled download scripts used by the latter.

It was pointed out in the review that this reimplementation and the original class could be refactored to change methods that are currently instance methods to static methods and remove some duplicated code.

These both could be further refactored into a common ancestor to further reduce duplication.